The vast majority of new designs today are drain back systems, to avoid antifreeze, and the stagnation related breakdown and corrosion problems it makes. Drain back systems are inherently freeze proof, and typically last twice as long. My system is the simplest of the drainback systems- I have a single 800 gallon, insulated drain back tank, mostly below grade. One pump runs/starts the system and if it fails, the water drains back into the tank and that's it. I use that water directly for my in floor heat, and domestic water is heated via copper coils in the tank. The tank is EPDM pond liner with 3" foam board, cement board, dirt. A 12" tall framed wall above and insulated, EPDM covered lid let me have the pump sit on the floor next to the tank (in my shop building).
Flat panels are much cheaper to build, but your maximum temperatures will be much lower. My flat panel collectors did have 20F higher temperatures in winter when they were double glazed with greenhouse polycarbonate panels, but the panels failed from mechanical stress- the difference in temperature of the inside and outside polycarbonate (very, very thin) was too great, causing a great deal of bending and buckling, especially during stagnation (tank maxed out so panel left sitting in the sun with no water flow). I replaced them after a few years with a single layer of 1/16th inch thick polycarbonate... which has performed marvelously. I do notice on the rare very cold, windy days that production is down perhaps 30%.
If you need higher water temperatures you have no choice but to use evacuated tubes. The flat panels just can't give you water temps in winter much over 110F, and often 100F. That's fine for my in floor system, which can heat the house just fine with 90F water.
The other option for larger hot water capacity at higher temperatures needed for radiators and such is parabolic trough collectors with the central tube in evacuated glass tube.
To recap: Flat panels are fabulously easy to build (see BuildItSolar.com for the copper pipe/aluminum fin designs like mine). But they are not designed to make high temperature water in winter. They work fabulously well with a well done in-floor heat system, which will work with low temperatures.